Dart-Shaped Tricrystal ZnS Nanoribbons.

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Nanostructures
DOI: 10.1002/ange.200504069
Dart-Shaped Tricrystal ZnS Nanoribbons**
Xia Fan, Xiang-Min Meng,* Xiao-Hong Zhang,* WenSheng Shi, Wen-Jun Zhang, Juan A. Zapien, ChunSing Lee, and Shuit-Tong Lee
Structural parameters such as size and shape have a significant influence on the properties of nanostructures, and
extensive efforts have been expended on controlling these
parameters during synthesis.[1, 2] Novel applications might
emerge if complex nanocrystals, such as three-terminal
ballistic junctions or Y-branch junctions,[3, 4] and well-defined
three-dimensional (3D) architectures could be synthesized in
a controllable manner.[5–7] To date, the shape/morphology
control typically relies on anisotropic nanocrystal nucleation
and growth from a liquid medium. Another standard
approach uses a static template and the differential growth
rates of different crystal facets. This makes possible the
controlled growth of branched inorganic nanostructures.[8, 9] In
comparison to the large amount of work on 1D nanostructures (wires and tubes), there are relatively few reports
on 3D nanostructures. Recently, some special 3D architectures, such as comb-, windmill-, and tetrapod-like, and
nanopropellor array structures, with nanometer and micrometer sizes have been observed in Group II–VI semiconductors (ZnO, CdS, CdSe, and ZnSe).[10–14]
ZnS, a wide direct-bandgap (3.68 eV at 300 K) II–VI
semiconductor, has been considered as a promising material
for the fabrication of blue electroluminescent (EL) diodes
and lasers.[15, 16] Many efforts have been made on the synthesis
of 1D ZnS nanostructures,[17–22] which have unique properties
for optical and electrical applications. We have reported the
synthesis of ZnS bicrystal ribbons and explained their growth
mechanism by combining the vapor–liquid–solid (VLS) and
[*] Dr. X. Fan,[+] Prof. X.-M. Meng, Prof. X.-H. Zhang, Prof. W.-S. Shi,
Prof. S.-T. Lee
Nano-organic Photoelectronic Laboratory
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences, Beijing 100101 (P.R. China)
Fax: (+ 86) 10-6487-9375
E-mail: xmmeng@mail.ipc.ac.cn
xhzhang@mail.ipc.ac.cn
Dr. W.-J. Zhang, Dr. J. A. Zapien, Prof. C.-S. Lee, Prof. S.-T. Lee
Center of Super-Diamond and Advanced Films (COSDAF)
and Department of Physics and Materials Science
City University of Hong Kong, Hong Kong SAR (China)
[+] Also at: Graduate School of Chinese Academy of Sciences
Beijing 100039 (P.R. China)
[**] We thank the Chinese Academy of Sciences and the CAS–Croucher
Funding Scheme for Joint Laboratories of the Croucher Foundation
for financial support and the National Natural Science Foundation
of China (No. 50572109). The work in Hong Kong was supported by
the Research Grants Council of Hong Kong SAR, China, through a
central allocation project (No. CityU 3/04C).
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oxide-assisted growth (OAG) models.[23] More recently, Zhu
et al.[24] reported the growth of ZnS architectures with core
rods covered with a spine-type array. Herein, we report a
simple thermal evaporation technique for the fabrication of a
novel ZnS architecture made of three intersecting ribbons
that form a dart-shaped tricrystal (DST) nanostructure.
The synthesis of dart-tail-like ZnS nanoribbons was
carried out in a vacuum tube furnace. An alumina tube was
mounted inside the tube furnace and used as the vacuum
chamber. Trace SiO powder was placed at the center of the
tube, and ZnS powder was placed 10 cm away (in the
downstream direction) from the SiO powder. Si wafers in
an alumina boat were placed near the downstream end of the
tube, which was sealed and evacuated to a pressure of 2 A
10 5 Torr. The furnace was then heated to 1100 8C at a rate of
approximately 10 8C min 1 and kept at this temperature for
4 h. Throughout the entire heating and cooling process, a
constant flow of high-purity Ar premixed with 5 % H2 was fed
into the tube at a flow rate of 50 standard cubic centimeters
per minute and a pressure of 30 Torr. After the furnace had
cooled to room temperature, the white products deposited at
around 900 8C on the inner wall of the alumina boat were
collected and characterized by scanning electron microscopy
(SEM; Hitachi S-4300FEG).
Figure 1 a reveals that the product is not primitive 1D
rods, but consists of 3D objects with a complex dart-tail-like
structure and a length of tens of micrometers. A highmagnification SEM image (Figure 1 b) clearly shows that the
Figure 1. a) SEM image of the ZnS dart-shaped tricrystal (DST) synthesized by simple thermal evaporation. b) Enlarged SEM image of the
end of a DST.
dart-tail-like structure is made of three intersecting ribbons
sharing a common “spine” over the entire length of the
structure, and each ribbon within this architecture has a width
of 0.5–1 mm. Notably, the deposition product consists almost
entirely of the DST structure. Nanostructures of other
morphologies, such as particles, were seldom observed.
X-ray diffraction analysis shows that the product has the
wurtzite (hexagonal) ZnS structure with lattice constants of
a = 0.382 and c = 0.626 nm (JCPDS: 36-1450). No characteristic peaks from other impurities, such as ZnO and Si, can be
detected from the XRD data, which suggests that the
deposition product has high phase purity.
The detailed microstructure and composition of the
product were characterized by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and
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energy-dispersive X-ray spectrometry (EDS). Figure 2 a
shows a bright-field image of the DST ZnS nanostructures,
with three intersecting ribbons sharing a common spine over
the entire length. Several tens of these structures have been
Figure 2. a) Bright-field TEM image of the dart-shaped nanoribbons.
Inset: arrow-shaped tip. b) SAED pattern of the DST marked with a
dark arrow in (a). c) Bright-field TEM image of the end of a DST.
d) HRTEM image of the region marked with a rectangle in (c).
e) SAED pattern and f) HRTEM image of the dart-shaped nanoribbon
after tilting by an angle of 308. g) EDS spectrum of a DST ribbon.
examined, and most of them have a diameter of approximately 500 nm, while DSTs with a diameter up to 1 mm were
occasionally observed. An arrow-shaped tip is shown in the
upper right inset in Figure 2 a. Figure 2 b is a SAED pattern
from the single DST nanoribbon marked by an arrow in
Figure 2 a. The pattern shows two sets of diffraction spots
having a twin relationship with a common (01̄3) plane, and
each set can be indexed with a [131] axis of a wurtzite ZnS
crystal. Figure 2 c shows one end of a DST ZnS nanostructure,
in which two branches stretch out their widest facets, while
the third protrudes vertically on the common spine. Figure 2 d
Angew. Chem. 2006, 118, 2630 –2633
is a high-resolution (HR) TEM image taken from the
interface region of the two nonvertical branch ribbons in
Figure 2 c. It can be seen that the boundary is uniform and
straight, and the angle between the two (101̄) planes of the
two branch ribbons is around 109.58.
To obtain further structural information on the ribbons,
the DST was analyzed in detail by a TEM tilting experiment.
The SAED of one of the intersection ribbons reveals a single
set of spots along the [100] zone axis of the wurtzite-2H ZnS
structure (Figure 2 e), which suggests that the flat surface of
the ribbon is along the (21̄0) plane. Figure 2 f is an HRTEM
image corresponding to Figure 2 e, in which stacking faults
can be seen along the (002) plane. The angle between the
(002) plane and the edge of the nanoribbon is about 178. The
growth direction of the ribbon was determined to be [2̄21̄].
Similar observations were obtained from the other ribbons in
this and a couple of other DSTs.
Figure 2 g is an EDS spectrum of a DST ZnS structure,
which reveals the presence of Zn, S, Si, and O (the Cu signal
comes from the TEM grid) with atomic ratios of 48:46:2:4.
Together with the above results, we deduce that the DST
structure consists of a ZnS wurtzite crystal with a trace
amount of amorphous silicon oxide on the surface.
Figure 3 shows a typical photoluminescence (PL) spectrum of the synthesized ZnS DSTs measured at room
temperature with Nd:YAG laser excitation at 266 nm. The
spectrum with a peak centered at 332 nm is similar to that of
ZnS nanowires.[21]
Figure 3. Room-temperature photoluminescence spectrum of the assynthesized ZnS DSTs.
SEM investigation reveals several kinds of morphology of
the ends of the DST ZnS nanostructures. Figure 4 a shows a
Figure 4. SEM images of ZnS DST nanoribbon: a) tip region, b,c) end
region.
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DST nanoribbon with a tetrapod tail, which directly indicates
the orientation relationship between three intersecting dartshaped ribbons. Figure 4 b shows an SEM image of the tip of a
DST nanoribbon, whose size gradually decreases from body
to tip. In Figure 4 c, a single nanowire with [001] growth
direction was found to grow from the tip of a DST nanoribbon.
In our experiment, the source is high-purity ZnS powder
with a sphalerite structure which has a sublimation point of
about 1020 8C. The growth direction of the DST is [2̄21̄],
which is different from the [120] growth direction of ZnS
nanoribbons synthesized by the vapor–solid (VS) method,[17]
and from the [121] and [131] directions of ZnS bicrystalline
nanoribbons and the [001] direction of ZnS nanowires
synthesized by the VLS method.[21, 22] Importantly, when SiO
is removed, no DST structure can be formed.
The growth mechanisms of the present ZnS DST nanoribbons are not fully understood, and therefore a preliminary
growth model is proposed here for discussion. On the basis of
the present observations, a combination of two different
processes, namely metal-catalyzed VLS growth and catalystfree OAG,[14, 25, 26] is used to explain the growth of the DST
structure. First, the ZnS source material reacts with H2 in the
carrier gas to form Zn and H2S vapors. Concurrently, the SiO
source sublimes to form SiO vapor. All vapors are then
transported by the carrier gas to a low-temperature zone and
deposited on the inner wall of the alumina boat. The SiO
vapor reacts with O2 (or H2O) to form SiOx vapor in the
transport process. The mixture of Zn and SiOx vapor forms
liquid droplets of solid solution in the low-temperature
region. The liquid droplets would continuously absorb more
Zn, H2S, and SiOx from the vapor. Inside the droplets, Zn and
H2S react to form ZnS and H2. SiOx decomposes into SiO2 and
Si. When the concentrations of ZnS, Si, and SiO2 reaches
supersaturation with respect to the local temperature, ZnS
would precipitate from the droplet and form the nanoscale
ZnS core.
It has been reported that nuclei are often in the form of
multiply twinned octahedrons during solidification of some
hexagonal materials such as ZnO and ZnSe.[27, 28] Selforganization in superstructures has also been proposed to
explain the formation of unusual geometries in solutions such
as PbSe.[29, 30] It is believed that the present DST structure
shares a similar initial nucleation mechanism to that of these
tetrapod nanostructures. The ZnS would first precipitate as
octahedrons (Figure 5 a) composed of twinned crystals.[31] As
a result of the polarity of ZnS, these octahedral nuclei would
then grow along the fastest growth direction to form a
nanoscale tetrapod (Figure 5 b). When the tetrapod grows to a
size similar to that of the original liquid droplet, surface
tension would draw the tetrapod to the side of the liquid
droplet. Effectively, the liquid droplet would now be captured
by three legs of the tetrapod (Figure 5 c). When more source
vapors are absorbed into the liquid, precipitation of ZnS
would continue by growth along the interface of the tetrapod
and the liquid droplet. This would lead to the formation of the
DST structure shown in Figure 5 d. SiO2 segregates from the
droplet and wraps on the surface of the DST structure.
Eventually, when the supply of the source vapors is terminated, further growth of the DST would reduce the size of the
liquid droplet. These steps explain the gradual size reduction
in the DST structure near the end (Figures 5 e,f and 4 b).
Finally, the liquid droplet would become too small to support
further growth of the DST. Depending on the local temperature, the tiny liquid droplet might simply solidify or grow
into a nanowire (Figures 5 g and 4 c) according to the VLS
mechanism.
In conclusion, dart-shaped ZnS semiconductor nanostructures made of three intersecting ribbons have been synthesized by a simple SiO-assisted thermal evaporation process.
The tricrystalline ribbons have a wurtzite-2H structure. Asgrown ZnS ribbons have lengths of several tens of micrometers, and each intersected ribbon within a DST structure
has a width of 500 nm to 1 mm. The growth direction of the
DST nanoribbons was observed to be [2̄21̄] of the ZnS
wurtzite structure. The room-temperature PL spectrum of the
DST nanostructure revealed a strong, sharp UV emission
band at 332 nm. The mechanism of formation of the DST
structures was explained by the production of tetrapod nuclei
followed by growth at the interface of the tetrapod and the
liquid catalyst.
Received: November 16, 2005
Revised: January 26, 2006
Published online: March 17, 2006
.
Keywords: crystal growth · electron microscopy ·
nanostructures · semiconductors · zinc
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Figure 5. Schematic diagram of the growth mechanism of ZnS DST
nanoribbons; see text for details.
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